Vibration arrangements of the kind referred to above are generally used in compactors for compacting soil, asphalt or the like. The amplitude of the vibratory movement can be fixed or it can be continuously adjusted as disclosed in U.S. Pat. No.4,481,835 entitled "Device for Continuous Adjustment of the Vibration Amplitudes of Eccentric Elements".
However, irrespective of whether the amplitude of vibration is fixed or adjustable, there has been a problem associated with the journalling of large eccentric forces. More specifically, the dimensions of the bearings are extremely large and there is the danger that the permissible shaft speed limits can be readily exceeded. To obviate this danger, it has been suggested to journal the rotational shaft of the eccentric member in two bearings on each side of the member. However, this solution brings with it the disadvantage that the bearing forces will be unevenly distributed because of the elasticity of the shaft.
Another way of solving the bearing problem is to journal two separate eccentric members in two bearings each at the respective ends of the common rotational shaft of the eccentrics. However, this is a comparatively expensive solution.
In connection with the configuration of bearing arrangements for an assembly with adjustable vibration amplitude, one solution would be to arrange the assembly in the form of two coaxial shafts fitted with respective eccentrics. A common version of such an assembly is a hollow shaft and a solid shaft arranged therein. Both shafts are provided with eccentric means, the hollow shaft being rotatably journalled on the inner solid shaft.
The disadvantage of this bearing arrangement is that the bearing of the inner shaft must be dimensioned for the maximum eccentric force. Furthermore, the eccentric shafts as a rule are stationary in relation to each other during vibration with the attendant danger of pitting readily arising in the bearing race of the bearings of the hollow eccentric shaft. If a plain bearing is used instead for the outer eccentric member to avoid such pitting, it will give rise to considerable friction and large forces for adjustment will result. Furthermore, the plain bearing requires good lubrication.